1. Field of the Invention
The present invention relates to methods and apparatus for forming a wellbore by drilling with casing. More specifically, the invention relates to a retrievable latch for connecting a bottom hole assembly to casing.
2. Description of the Related Art
In well completion operations, a wellbore is formed to access hydrocarbon-bearing formations by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a drill support member, commonly known as a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annular area is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. A cementing operation is then conducted in order to fill the annular area with cement. The casing string is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole using apparatuses known in the art. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with a drill bit on a drill string. The drill string is removed. A first string of casing or conductor pipe is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string may then be fixed, or “hung” off of the existing casing by the use of slips which utilize slip members and cones to frictionally affix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
As more casing strings are set in the wellbore, the casing strings become progressively smaller in diameter to fit within the previous casing string. In a drilling operation, the drill bit for drilling to the next predetermined depth must thus become progressively smaller as the diameter of each casing string decreases. Therefore, multiple drill bits of different sizes are ordinarily necessary for drilling in well completion operations.
Well completion operations are typically accomplished using one of two methods. The first method involves first running the drill string with the drill bit attached thereto into the wellbore to drill a hole in which to set the casing string. The drill string must then be removed. Next, the casing string is run into the wellbore on a working string and set within the hole. These two steps are repeated as desired with progressively smaller drill bits and casing strings until the desired depth is reached. For this method, two run-ins into the wellbore are required per casing string that is set into the wellbore.
The second method of performing well completion operations involves drilling with casing. In this method, the casing string is run into the wellbore along with a drill bit, which may be part of a bottom hole assembly (BHA). The BHA is operated by rotation of the casing string from the surface of the wellbore or a motor as part of the BHA. After the casing is drilled and set into the wellbore, the first BHA is retrieved from the wellbore. A smaller casing string with a second BHA attached thereto is run into the wellbore, through the first casing. The second BHA is smaller than the first BHA so that it fits within the second, smaller casing string. The second, smaller BHA then drills a hole for the placement of the second casing. Afterwards, the second BHA is retrieved, and subsequent assemblies comprising casing strings with BHAs attached thereto are operated until the well is completed to a desired depth.
One problem noticed in drilling with casing operations is attaching and retrieving the drill bit from the wellbore. In conventional methods, the drill bit is fixably attached to the end of the casing and must be drilled-out using a subsequent casing and drill bit assembly. In other conventional methods, the drill bit is attached to the casing using a retrievable latch. However, a problem that arises using a latch assembly is that foreign matter or debris can prevent or impede either the activation or retrieval of the latch. For example, foreign matter may become lodged or wedged behind expanded components that must be retracted for the latch to disengage from the surrounding casing. In these instances, in order to resume drilling operations, the BHA must be retrieved from the hole, replaced, and run back in, consuming valuable time and generating cost.
Another problem noticed with existing retrievable latches is their complexity. The complexity of these latches may result in low reliability and high cost. Further, these complex designs may require multiple steps to disengage the latch from the casing.
Therefore, a need exists for a latch that attaches a BHA to a casing string, which can be reliably activated and retrieved from the wellbore. There is also a need for a latch that prevents foreign matter and debris from impeding or preventing its intended operations. Further, there is a need for a relatively simple latch that may easily be disengaged from the casing.